It has been desired to develop an optical element that is compact and consumes a low power. In such an optical element, using a minute ring modulator with a silicon sub-micron optical waveguide has been studied.
FIG. 9 is a diagrammatic view depicting a schematic configuration of an optical element of related art using a ring modulator.
The optical element includes a distributed feedback (DFB) laser 101, a ring modulator 102, a PD 103, a wavelength controller 104, and a heater 105.
The PD 103 senses power of light having passed through the ring modulator 102. The wavelength controller 104 outputs a signal that controls the wavelength of laser light based on the optical power sensed with the PD 103. The heater 105 heats the ring modulator 102 in accordance with the control signal from the wavelength controller 104 to adjust the wavelength of the ring modulator to match the laser wavelength.
In the optical element, the DFB laser 101 outputs laser light in a continuous emission mode, and this laser light is guided to the ring modulator 102, which modulates the transmissivity at the laser light wavelength transmissivity.
The ring modulator 102 has a Lorentzian transmission spectrum centered at a resonance wavelength, and changes the resonance wavelength in accordance with a change in a modulation signal between voltages V0 and V1. The transmissivity is thus modulated, whereby intensity-modulated output light is produced.
The resonance wavelength of the ring modulator 102 changes as a circumference optical path length of the ring modulator 102 changes due to an error in manufacturing the ring modulator 2 and/or a temperature change, resulting in a discrepancy between the resonance wavelength and the wavelength of the laser light being emitted. As depicted in FIG. 10, to compensate for the discrepancy, the heater 105 heats the ring modulator 102 to raise the temperature of it for adjustment of the resonance wavelength.
In this case, however, it is undesirably difficult not only to ensure reliability of the optical element but also to improve power efficiency in the wavelength adjustment mechanism and modulation operation for the transmissivity (decrease in electric power necessary for the heater and modulation operation). The reason for this is as follows.
As depicted in FIG. 11A, reducing the radius of the ring modulator 102 reduces the volume of the ring modulator 102, whereby heater power required to compensate for the wavelength shift resulting from variation in temperature is decreased. Furthermore, reducing the radius of the ring modulator 102 reduces a capacitance that serves as a load when viewed from a drive circuit of the ring modulator 102, whereby the modulation power is deceased. On the other hand, because the difference between the laser and the ring modulator wavelength amounts up to the free spectral range (FSR), an increased FSR increases the amount of required wavelength compensation, resulting in an increase in the amount of increase in the temperature of the ring modulator 102 and hence a decrease in reliability of the ring modulator 102.
As depicted in FIG. 11B, increasing the radius of the ring modulator 102 reduces the FSR, resulting in a decrease in the amount of wavelength compensation, which reduces the amount of increase in the temperature of the ring modulator 102, whereby reliability of the ring modulator 102 is ensured. On the other hand, the volume of the ring modulator 102 increases, resulting in increases in modulation power, and heater power required to compensate for the wavelength shift due to variation in temperature.
The technique disclosed in Patent Document 1 has been devised in order to address the described problems. Patent Document 1 discloses the technique in which the resonance wavelength of a ring modulator causes laser oscillation. A discrepancy between the resonance wavelength of the ring modulator and the laser wavelength is thus at most an interval between Fabry-Perot (FP) modes. This enables a significant decrease in the amount of wavelength compensation as compared with the related art that was required to compensate for the wavelength shift by the free spectral range (FSR) at the maximum, whereby a significant effect for the decrease in temperature increase, namely for the improvement in reliability is exhibited.
In the case of Patent Document 1, however, an intensity-modulated optical signal is dispersed to two ports, thus to cause a problem that the power of output light per one port decreases by half.
Patent Document 1: Japanese Laid-open Patent Publication No. 2015-230991
Patent Document 2: Japanese Laid-open Patent Publication No. 2012-64862
Patent Document 3: Japanese Laid-open Patent Publication No. 2009-59729